The following manuscripts (and one by Malan elsewhere in this volume) will consider how hibernators reduce metabolism and initiate cooling. This contentious issue has fascinated students of hibernation since the first symposium in 1959 (see Morrison, 1960). At the time of that symposium, it was recognized that Q₁₀ of various processes varied during entry into hibernation: abandoned thermoregulation (2.5-fold), tissue Q₁₀ effects (10-fold), unspecified metabolic depression (3-fold), with an overall Q₁₀ of 6 for the entire entry into hypothermia from 30 to 6 C, followed by a further temperature-independent depression (1.8-fold) (Morrison, 1960). Currently, the debate focusses on whether metabolism is depressed due to suppression by acidosis, by a Q₁₀ effect associated with the progressive reduction of body temperature, by other unknown factors, or by some combination of factors.